Figure 3.14. Change in Manning's n with discharge for Padma River in Bangladesh.
3.4.12 How Bedform Changes Affect Highways in the River Environment
At high flows, most sandbed channel streams shift from a dune bed to a transition or a plane
bed configuration. The resistance to flow is then decreased two to threefold. The
corresponding increase in velocity can increase scour around bridge piers, abutments, spur
dikes or banks and also increases the required size of riprap. On the other hand, the
decrease in stage resulting from the planing out of the bed will decrease the required
elevation of the bridge crossing, the height of embankments across the floodplain, the height
of any dikes, and the height of any channel control works that may be needed; and the
converse is also true.
Another effect of bed form on highway crossings is that with dunes on the bed there is a
fluctuating pattern of scour on the bed and around the piers, abutments, guide banks and
spurs. The average height of dunes is approximately 1/2 to 1/3 the average depth of flow
and the maximum height of a dune may approach the average depth of flow. If the depth of
flow is 3 m (10 ft), the maximum dune height may be of the order of 3 m (10 ft), and half of
this would be below the mean elevation of the bed. With the passage of this dune through a
bridge section, an increase of 1.5 m (5 ft) in the local scour would be anticipated when the
trough of the dune arrives at the bridge.
A very important effect of bed forms and bars is the change of flow direction in channels. At
low flow the bars can be residual and cause high velocity flow along or at a pier or abutment
or any of the other structures in the stream bed, causing deeper than anticipated scour. As
stated previously large discharges normally experience smaller resistance to flow in a
sandbed stream due to the change in bed form. However, if the bridge crossing or
encroachment causes appreciable backwater, the dune bed may not plane out at large
discharges and a higher resistance to flow results. This increase in resistance to flow can
decrease the velocity of flow and also decrease the transport capacity of the channel so that
aggradation occurs upstream of the crossing. The aggradation and the roughness increases
the river stage and thus the height of any control structure or the levees. Thus, the bridge
crossing can adversely affect the floodplain, due to the change in bed form that would occur.